Membrane processes are material separation processes based upon the passage of material through the membranes wall to allow the selective separation of different materials.
A membrane is a solid configured as a thin layer of large surface area. Among other purposes, membranes serve as tools of separating two or more material components out of a mixture. The separative properties of membranes are based upon their different permeabilities to the various material components. Thus, in order to effect a separation, a driving force must exist across the membrane, which promotes the permeation of material through it, which may be a pressure or a concentration differential.
For the case of pressure differential driving force, the membrane must acquire the mechanical strength necessary to withstand the pressure differential. This contradicts the requirement that the membrane must be as thin as possible in order to allow large cross-flows. Therefore, the membrane is supported on a relatively porous support which sustains the mechanical load on the one hand, and allows free cross-flow of material through its large pore system on the other hand.
The necessity to have thin membrane layers, large surface areas in a given volume of a membrane separator, and a porous mechanical support has brought about the development of the hollow fiber membranes. These are thin tubes of circular cross-section. As such, they are self-supporting and occupy a small volume per unit surface area. A hollow fiber membrane module for separation is most frequently designed as a shell-and-tube assembly, as shown in FIG. 1. In such an arrangement, separate accesses to the interior (bore) and exterior (shell) sides of the HF membrane are possible, so as to enable the separation of the three major material flows: the feed, the reject which is collected at the opposite side of the module but at the same membrane face at which the feed was introduced, and the permeate which is the result of the flow through the membrane.
A bundle of hollow fiber (HF) membranes, when assembled in a shell-and-tube configuration for separation purposes, may contain a few broken or pinholed fibers. This is schematically illustrated by numeral 1 in FIG. 2, where a broken fiber is shown. The material transfer through these defects is non-selective and very high in comparison to the rest of the membrane wall, which was designed to acquire a desired selectivity. Therefore, such defects destroy the separative properties of the membrane bundle, and, if they are frequent, practically render it ineffective. It is therefore clear that, in order to be able to exploit fiber bundles containing failed fibers, it is necessary to find a method by means of which these defects can be repaired or avoided, or their undesirable result excluded.
The obvious method to avoid such defects is to improve the various steps of the process of membrane production. However, spinning hollow membrane fibers at an industrial scale always suffer from some extent of defects. This is especially so in the case of asymmetric membranes which have a very thin and thus vulnerable active skin, whereas the occurrence of pinholes of various diameters is very likely. In fact, the limit to thinning the skin in hollow fibers is set by the formation of defects.
Other occurrences of pinhole formation in membranes result not in the course of membrane production, but in the course of subsequent handling and treatments. This is particularly abundant in membranes of great brittleness and fragility, as is the case with ceramic and carbon membranes, and certain stiff, glassy polymers. Under such circumstances, the hollow fibers produced by the spinning system may be free of defects, but further handling which asserts mechanical stress, such as spool winding and unwinding, cutting the fibers to the length, inserting the hollow fiber bundle into the tube and end potting may cause damage.